Python 3.5.1 (v3.5.1:37a07cee5969, Dec  5 2015, 21:12:44)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
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>>> class Descriptor(object):
...     def __init__(self, name):
...         self.name = name
...     def __set__(self, instance, val):
...         print('Updating', self.name, 'for', instance)
...         instance.__dict__[self.name] = val
...
>>> class A:
...     x = Descriptor('x')
...     def __init__(self, name, x):
...         self.x = x
...         self.name = name
...     def __repr__(self):
...         return "I am {}".format(self.name)
...
>>> a = A('a', 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
  File "<stdin>", line 5, in __set__
  File "<stdin>", line 7, in __repr__
AttributeError: 'A' object has no attribute 'name'
Updating x for >>>
>>> # if I just turn around the assignments of self.x and self.name in A,
... # everything works as expected:
...
>>> class A:
...     x = Descriptor('x')
...     def __init__(self, name, x):
...         self.name = name
...         self.x = x
...     def __repr__(self):
...         return "I am {}".format(self.name)
...
>>> a = A('a', 2)
Updating x for I am a
>>> a.x
2